1. Field of the Invention
This invention relates to the field of forming contacts and contact openings for semiconductor devices.
2. Prior Art
In the processing of semiconductor devices, it is desired to form conductive pads to make contact to the devices. Particularly with transistors, the gate, source and drain must all have contacts. Typically, transistors are three-dimensional with the gate area higher than the surrounding source/drain area. In order to reduce coupling capacitance and prevent shorting of metallurgy to source, drain, and gate regions, the entire device is covered with an insulative dielectric layer. Openings are then formed in this insulative layer over the source of drain and gate and a metal layer is formed therein for contacts.
In the prior art process, the gate, source and drain areas of a field effect transistor are reoxidized. Then a layer of phosphorus doped glass is deposited. The doped glass is subjected to a thermal cycle so that it reflows to smooth corners for good metal coverage. Contacts are then patterned using standard photolithographic techniques. The phosphorus doped glass is wet etched and the thermally grown oxide is anisotropically dry etched. In this prior art process the contacts must be placed sufficiently away from the gate so as to prevent exposing the thermal oxide on a gate to the isotropic wet etch. If the thermal oxide is exposed by the wet etch, it is not sufficient to insulate the gate from the metal.
Therefore, it is an object of the present invention to provide a means of forming a contact opening which results in a gradual slope between elevations.
It is a further object of the present invention to provide a means of forming contacts near gates which still result in contact isolation and without a large increase in the required oxide to silicon etch selectivity.